I don't think you know much about Marx or Keynes, if you conflate them like this. Keynes has been proven right as thoroughly as it is practically possible to prove an economist right. The greatest economic expansion in the modern world happened during the poswar period in the US, western European and certain Asian economies when those economies were entirely under the influence of Keynesian policies. When Keynesian policies started being moved away in favor of Friedman's monetarist policies, in the mid 80's growth promptly slowed.

South America rejected Keneysian policies for what they believed were more "free market" policies. They suffered greatly, and they did not experience the massive postwar growth the Western world and the Asian Keynesian countries (Japan, Taiwan, S Korea) experienced. This was a great tragedy, because south america should have done great as they were the only corner of the world not affected by WW2. But they remained mired in poverty.

It is difficult to prove an economist right or wrong because it is difficult to run experiments in macro economics. But as much as it is possible to prove an economist right, Keynes has been proven right.